1. Field of the Invention
The present invention relates to a potentiometer wheel controller that provides a digital output corresponding to the position of the wiper of a potentiometer. The invention is of particular use in connection with thumb wheels used to control such functions as pitchbend and vibrato in digital music synthesizers.
2. Background Information
A typical music synthesizer may include a pair of thumb wheel controllers, one for pitchbend and the other for modulation functions such as vibrato and the like. Each of these controllers comprises a thumb wheel mechanically coupled to a potentiometer. A voltage is applied across the potentiometer and the resulting voltage at the moveable contact, or wiper, of the potentiometer is converted to a digital output, usually according to a MIDI (Musical Instrument Digital Interface) format. In order to compensate for such factors as tolerance variations in various components, the voltage applied to the potentiometer is also the reference voltage used by the analog-digital converter (ADC) that converts the potentiometer wiper voltage to a digital representation. However, a typical thumb wheel mechanism usually rotates the potentiometer through only a fraction of the rotational range of the wiper. For example, the thumb wheel itself may be readily rotational through only a ninety-degree angle corresponding to 30-33 percent of the total potentiometer range. Thus, given the use of an eight-bit converter to minimize costs, the thumb wheel range corresponds to only around 80 steps of the 256-step range of the converter. This is too coarse for many purposes, especially when controlling pitch.
In order to xe2x80x9cstretchxe2x80x9d the rotational range of the potentiometer to cover the full range of the ADC, one might use a custom potentiometer whose resistance element has been designed to cover only ninety degrees of rotation. However, the tooling expense and quantity commitment for such a part are not practical for applications where relatively small numbers are required. Another possibility is to increase the voltage across the potentiometer. However, this requires a second highly-regulated reference voltage. Furthermore, it may result in overdriving the ADC. A third method, which is currently in use, is to amplify the voltage swing from the potentiometer so as to cover the range of the ADC. If a true xe2x80x9crail-to-railxe2x80x9d amplifier is used and is powered by the reference voltage, there is no possibility of overdriving the ADC.
A thumb wheel used for a pitchbend ordinarily is connected to a spring mechanism that returns the wheel to its center position. It is important that when the wheel is released and returned by the spring mechanism, the converted digital output be returned exactly to its center value. Similarly, if the thumb wheel is set up for dual modulations, it is provided with a mechanical detent which ideally holds the control at its exact center where both the modulations are to be zero. With an 8-bit ADC the exact digital center is 128 for the usual case of unipolar coding. Mechanical or electrical centering methods can be employed to make this nominally true, but even a small amount of mechanical backlash, play or electrical drift will produce a center error of one bit or more.
In order to cope with mechanical and electrical tolerance variations while insuring that the mechanical center of the thumb wheel corresponds with the digital center, software is provided to implement a deadband in the middle of the digital range. For example, with an 8-bit ADC the software might consider any reading between 124 and 132 as the center, instead of only the center value 128. This provides a deadband tolerance of xc2x14 bits around the center value. The total range each side of center is then 123 steps instead of 127.
This results in a problem when dual modulation is implemented with a thumb wheel. The MIDI modulation values are typically represented by seven bits, corresponding to 128 steps. When a 123-step reduced wheel range is scaled to cover the 128 step MIDI modulation range, some one-bit thumb wheel movements result in 2-bit MIDI modulation changes while others result in a single-bit modulation change, resulting in a rough change in modulation level in response to smooth rotation of the thumb wheel.
A thumb wheel controller incorporating the invention uses analog circuitry to implement a center-position deadband. Specifically, it includes a pair of operational amplifiers biased so that one covers a lower thumb wheel range segment below the deadband and the other covers an upper range segment above the deadband. The gain of each amplifier is such that its full range of output voltages corresponds to the full lower (or upper) thumb wheel range segment. A single analog-digital converter may be multiplexed to the outputs of the two amplifiers and thus a single ADC can provide a full representation of thumb wheel position in each of the lower and upper range segments, resulting in a 512-step (9-bit) representation of the full range of thumb wheel rotation.